Hydrogen is a potential clean energy source for a variety of applications, such as automobile applications. Hydrogen includes about three times the energy of gasoline by mass and produces water as a by-product of combustion. However, due to its low density in gas or liquid form, hydrogen includes significantly less energy per unit volume relative to gasoline. In order to carry a sufficient amount of hydrogen, an automobile has to allocate a large amount of valuable space for a high pressure tank or a cryogenically cooled storage tank. Therefore, a barrier for commercialization of hydrogen-powered automobiles is a hydrogen storage system with a volumetric density greater than that of liquid hydrogen. Desirably, the storage system should also have a high gravimetric capacity, such as in terms of weight percent of hydrogen relative to a total weight (or wt. % H2), have fast kinetics, and operate under moderate temperatures and pressures over hundreds of cycles.
Solid-state hydrogen storage materials have the potential to meet desired criteria for a hydrogen storage system. Materials such as LaNi5H6 and TiFeH2 reversibly store hydrogen at volumetric densities superior to liquid hydrogen and under moderate temperatures and pressures. However, the gravimetric capacities of these materials are considered too low for typical automobile applications. Other materials, such as NaAlH4 and other complex metal hydrides, have higher gravimetric capacities, but suffer from significant kinetic barriers to hydrogenation and dehydrogenation. Such kinetic barriers, in turn, can adversely impact a fueling time of hydrogen-powered automobiles.
One of the most promising classes of solid-state hydrogen storage materials is based upon hydrogenation and dehydrogenation of imides and amides. In particular, hydrogenation and dehydrogenation in the so-called Mg—N—H system occur between a mixed imide, namely Li2Mg(NH)2, and an amide and a hydride, namely LiNH2 and MgH2. Li2Mg(NH)2 can absorb up to about 5.5 wt. % H2 with an enthalpy of hydrogenation of about −36 kJ/mol H2. This enthalpy change corresponds to an equilibrium hydrogen pressure of about 1 bar at about 90° C., which is near-optimal for typical automobile applications. Despite its highly desirable thermodynamics, the Mg—N—H system suffers from a significant kinetic barrier, such that, even at higher temperatures in excess of about 180° C., hydrogenation can be incomplete after several hours. In addition, decomposition with ammonia evolution can be promoted at these higher temperatures. The combination of incomplete hydrogenation and dehydrogenation and ammonia evolution can contribute to an undesirable amount of capacity loss when operated over several cycles.
It is against this background that a need arose to develop the hydrogen storage materials and related methods and systems described herein.